Speciation Analysis and Removal of Heavy Metals Zn, Cu, Cd from Sludge by Organic Acid

摘要

A chemical extraction procedure for removing extractable heavy metals Zn, Cu, Cd using organic acid as extractant has been applied to wastewater sediments collected from wastewater treatment plant. The Tessier's five-stage sequential extraction procedure for extractable heavy metals determination is applied to sludge samples. The results show that the extractable content of tested heavy metals exhibits a positive correlation with increasing acid concentration, reaction temperature and time. After the extraction process, citric acid is more efficient in removing Zn, oxalic acid exhibits higher extractability in removing Cu but inefficient in Zn and Cd. Zn removed by citric acid is mainly in stable form, resulting in a relative increase of its unstable and exchangeable form in sludge. Though the removal rate of Cu and Cd are lower, their exchangeable form has still increased after the action of tested organic acid. The analysis of heavy metals is of paramount important to assess soil pollution problems associate with sludge coming from wastewater treatment plants. Untreated sludge enters to environment will bring secondary pollution and pose a serious threat for ecological environment. However, the determi-nation of total concentration of heavy metals does not give an accurate estimation of potential environmental impact. It is crucial that heavy metals are removed or transferred to other active speciation with less biological toxicity. For this reason, chemically polarity method has become of growing importance in repairing sludge polluted by heavy metals. Among chemical extractant, organic acid has been widely studied. According to Tessier's work, heavy metals are extracted into exchangeable form, carbonates, iron/manganese oxides, substances/sulfide and residuals. Exchangeable form, carbonates, iron/manganese oxides are unstable state. Substances/sulfides and residuals form are stable state. The environmental effects of above chemical forms are as follows: exchangeable form is more sensitive to environmental changes, which is easy to migrate, transfer and be absorbed by plants. Carbonates form with more sensitivity to pH value may co-precipice with the increase of pH, and is easy to rerelease to environment. When pH value of soil decreases, iron/manganese oxides, substances and sulfides are relatively stable, but can be released and absorbed by biology. The difference lies in iron/manganese oxides show higher biological effectiveness in reducing condition, but substances and sulfides exhibit more activities under oxidizing condition. The metal in residue state is stable, and is uneasy to release or be absorbed by plants under normal condition. In the present work, Extraction process is conducted with organic acid to investigate the rules of speciation and transformation of Zn, Cu and Cd in sludge. The environmental behavior and biological toxicity of tested sludge are studied indicating the suitability of sludge samples for agricultural use in relation to current international legislation.